11 research outputs found
Confinement of superconducting fluctuations due to emergent electronic inhomogeneities
The microscopic nature of an insulating state in the vicinity of a
superconducting state, in the presence of disorder, is a hotly debated
question. While the simplest scenario proposes that Coulomb interactions
destroy the Cooper pairs at the transition, leading to localization of single
electrons, an alternate possibility supported by experimental observations
suggests that Cooper pairs instead directly localize. The question of the
homogeneity, granularity, or possibly glassiness of the material on the verge
of this transition is intimately related to this fundamental issue. Here, by
combining macroscopic and nano-scale studies of superconducting ultrathin NbN
films, we reveal nanoscopic electronic inhomogeneities that emerge when the
film thickness is reduced. In addition, while thicker films display a purely
two-dimensional behaviour in the superconducting fluctuations, we demonstrate a
zero-dimensional regime for the thinner samples precisely on the scale of the
inhomogeneities. Such behavior is somehow intermediate between the Fermi and
Bose insulator paradigms and calls for further investigation to understand the
way Cooper pairs continuously evolve from a bound state of fermionic objects
into localized bosonic entities.Comment: 29 pages 9 figure
Spectroscopic evidence for strong correlations between local superconducting gap and local Altshuler-Aronov density-of-states suppression in ultrathin NbN films
Disorder has different profound effects on superconducting thin films. For a
large variety of materials, increasing disorder reduces electronic screening
which enhances electron-electron repulsion. These fermionic effects lead to a
mechanism described by Finkelstein: when disorder combined to electron-electron
interactions increases, there is a global decrease of the superconducting
energy gap and of the critical temperature , the ratio
/ remaining roughly constant. In addition, in most films an
emergent granularity develops with increasing disorder and results in the
formation of inhomogeneous superconducting puddles. These gap inhomogeneities
are usually accompanied by the development of bosonic features: a pseudogap
develops above the critical temperature and the energy gap
starts decoupling from . Thus the mechanism(s) driving the appearance of
these gap inhomogeneities could result from a complicated interplay between
fermionic and bosonic effects. By studying the local electronic properties of a
NbN film with scanning tunneling spectroscopy (STS) we show that the
inhomogeneous spatial distribution of is locally strongly correlated
to a large depletion in the local density of states (LDOS) around the Fermi
level, associated to the Altshuler-Aronov effect induced by strong electronic
interactions. By modelling quantitatively the measured LDOS suppression, we
show that the latter can be interpreted as local variations of the film
resistivity. This local change in resistivity leads to a local variation of
through a local Finkelstein mechanism. Our analysis furnishes a purely
fermionic scenario explaining quantitatively the emergent superconducting
inhomogeneities, while the precise origin of the latter remained unclear up to
now.Comment: 11 pages, 4 figure